Oil shale is a natural sedimentary rock containing an abundance of residual organic material which, when processed, can be made into oil and fuel products. Typically, oil shale, such as exemplified by the Green River formation in Wyoming, Colorado and Utah, has about 15-20% organic material embedded in an inorganic mineral matrix. The organic portion is composed generally of a soluble bitumen fraction in which kerogen constitutes the bulk of the insoluble organic material. In order to remove the organic material from the oil shale, the usual process has comprised crushing the matrix rock and subjecting the crushed matrix to heat in a retort to distill off the kerogen by destructive pyrolysis. A major waste product from oil shale retorting processes is water co-produced with the shale oil and which can also include a small amount of water originally trapped within the matrix of the oil shale. The amount of retort water formed may equal 20 to 100 barrels per 100 barrels of shale oil produced. Of course, the actual amount of retort water produced depends on the type of operating conditions of the retorting process as well as the location and nature of the oil shale.
Although water is produced as a result of retorting of the oil shale, in the production of oil from oil shale, a great deal of water is in fact used. For example, water is needed for fracturing during the mining operations and is used in kerogen concentration, in retorting, in upgrading the oil, in abatement procedures, etc. For each gallon of oil produced, about one gallon of water is needed with generally less water being required for in situ retorting processes as compared to above-ground processes. It would therefore be desirable to be able to utilize water formed from retorting for these production processes. However, the oil shale retort water is loaded with considerable quantities of soluble organic and inorganic materials not only preventing its utilization for production purposes, but also presenting a serious major disposable problem. Since economic feasibility is often called into question when one compares the cost of deriving useful oil products from oil shale, any basis for recovering of industrially usable water from the retort water constitutes a significant advantage.
A number of processes have been suggested for the treatment of retort water, including chemical precipitation, solvent extraction, ion exchange and limestone neutralization. Except for special circumstances, there are practical limitations in the applications of all the foregoing methods. Moreover, such methods generally require continued addition of treatment chemicals which increases the quantity of contaminants, complicates the problem of ultimate disposal, and decreases the opportunity for economic recovery.
The present invention provides a process for purifying oil shale retort water which does not suffer the drawbacks of the above-mentioned procedures. Specifically, the invention takes advantage of the fact that the oil shale retort water has a substantial amount of organic content and sufficient inorganic components, including sulfate, to enable the growth of anaerobic bacteria. Such bacteria, exemplified by the Desulfovibrio family, is added to the retort water so as to produce in the water a growth of cell biomass of the bacteria while simultaneously reducing sulfate ions to sulfide. Production of cell biomass should proceed substantially in the absence of air, preferably following purging with carbon dioxide, nitrogen or the like, to establish aerobiasis. The cell biomass is aggregated into a flocculent mass and is removed from the retort water.
The sulfide is oxidized to sulfate either through combustion processes or by conversion with air and aerobic bacteria, such as of the Thiobacillus family. At least a portion of the sulfate thus produced can be recycled to the retort water so as to neutralize the retort water prior to bacterial growth. Excess sulfate can be neutralized, for example, by reaction with spent shale, that is, by reaction with the inorganic matrix of retorted oil shale. The effluent discharge from this treatment yields a water quality which not only reduces environmental damage, but also supplements industrial water requirements of the mining and retorting operation. The reacted spent shale can be combined with the removed cell biomass and used as fertilizer. Alternatively, a portion, or all of the cell biomass, can be recycled to the retort water as nutrient for growth of the bacteria.